[kiwi_steve]

Glad it was something simple and inexpensive

My gut feeling is that the info you're working from has a typo - because the readings you're getting are what I would expect... there is no path I can see to measure resistance from the negative lead of that cap to the plate of the output tube other than through the electrolytic itself.

Regarding hum - 40uF is the max rated smoothing cap for the 35Z5 - so you could try increasing C101A to 40 if you can find that value... alternatively have you moved any of the wiring or component placements?

Cheers

Steve